1. Field of the Invention
The present invention relates to a heating roller arranged in a fixing device used in an image forming apparatus such as an electrophotographic apparatus, etc.
2. Description of the Related Art
A fixing method used in an image forming apparatus is classified into three systems of thermal fixation, pressure fixation and solvent fixation. The thermal fixation is used in an electrophotographic apparatus. For example, in the structure of a fixing device for performing such thermal fixation, a heating roller and a pressurizing roller are arranged such that the heating and pressurizing rollers are opposed to each other through a paper sheet conveying path and are rotatably supported and come in press contact with each other by a spring, etc. Toner transferred onto a sheet of printing paper is heated and melted by heat of the heating roller. The pressurized and melted toner is fixed onto the printing paper sheet when the paper sheet is conveyed between the heating and pressurizing rollers. Accordingly, the heating roller simultaneously heats and conveys the printing paper sheet so that the image forming apparatus can be made compact and light in weight.
In many cases, the heating roller generally has a structure in which a heating resistor formed in the shape of a shaft is arranged within a metallic roller having a circular tube shape. However, thermal efficiency of the heating roller is low in this structure so that it is difficult to save power and improve responsibility, etc. Therefore, for example, Japanese Patent Application Laying Open (KOKAI) No. 55-164860 proposes a fixing device of a surface heating type in which a surface of the heating roller is formed by a heating resistor.
In this proposed fixing device, the surface heating roller is constructed such that a pair of electrode members of a power receiving portion are arranged on outer circumferential faces of both end portions of the surface heating roller. This surface heating roller is arranged on a paper sheet conveying path such that the surface heating roller is rotatably supported between body frames. A pair of electricity-flowing brushes of a transmission section connected to a power source on an unillustrated body side of the fixing device are resiliently mounted to the body frames by springs respectively arranged on inside faces of the body frames. The electricity-flowing brushes respectively come in contact with the electrode members of the surface heating roller.
In the fixing device having such a construction, the electricity-flowing brushes respectively come in contact with the electrode members arranged through electric conductors on the outer circumferential faces of the heating roller by resilient force of each of the springs. Therefore, power is supplied from the body power source to the rotating heating roller. The heating roller is heated by this supplied power so that toner on a printing paper sheet sequentially conveyed by rotating the heating roller is heated and fixed onto this printing paper sheet.
In many cases, the heating resistor, the electrode members and the electric conductors of the heating roller are formed by different materials. Differences between coefficients of thermal expansion of these constructional elements are relatively large. For example, when the heating resistor is mainly constructed by Al.sub.2 O.sub.3 having dispersed NiCr ranging from 25 to 30%, the coefficient of thermal expansion of this heating resistor is equal to 9.times.10.sup.-6 /.degree. C. When the electrode members are constructed by a CuAl alloy, the coefficient of thermal expansion of each of these electrode members is equal to 20.times.10.sup.-6 /.degree. C. Accordingly, there is a fear of causing a crack between a heating resistor layer and the electric conductors, between the electric conductors and the electrode members, or between the heating resistor and the electrode members on the heating roller repeatedly heated and cooled. Therefore, there are fears of generation of a spark and a conductive break by discharge from a portion of this crack. In particular, such fears are strong in an environment in which a high voltage power source is used in countries such as Europe, United States of America, etc.